We are sorry to let you know that neither of the relative ______

We are sorry to let you know that neither of the relative _______has reached us yet.
A、letter of credit
B、letter of credits
C、letters of credit
D、letters of credits
【正确答案】：C
【题目解析】：letter of credit信用证  复数形式为letters of credit
句意：我们很遗憾地通知你方，我们还没有收到有关信用证。